1. Field of the Invention
The present invention relates to an inkjet printing apparatus that performs printing on a printing medium by discharging ink drops from a nozzle of a printing head based on print data, and an ink discharge control method in the inkjet printing apparatus.
2. Description of the Related Art
Recent years have seen an increase in the number of colors, an increase in density, a decrease in the size of drops, and an increase in the number of nozzles in inkjet printing apparatuses in order to meet demand for even higher quality and higher speed than that in commercially available inkjet printing apparatuses. As a result, it has become possible to provide users with photograph images that are in no way inferior to even silver halide photographs in the case of performing printing on special media, in addition to applications for printing web pages and text on normal paper. Meanwhile, inkjet printing apparatuses for business use and industrial use with printing speeds that have been raised to the level of laser beam printers are widely prevalent in the market.
In order to raise the printing speed of such inkjet printing apparatuses for business use and industrial use, it has been common to elongate the nozzles of the printing head. With such inkjet printing apparatuses, it is difficult to maintain a constant distance between the nozzle face of the printing head and the printing medium (hereinafter, referred to as the “paper distance”). This is due to an increase in the distance from the pinch roller that supports the printing medium upstream of the printing head to the paper discharge roller that supports the printing medium downstream of the printing head. Accordingly, an electrostatic adsorption conveyance system has been realized in which, in a printing medium conveying mechanism using an endless belt, a printing medium is adsorbed to the endless belt by generating static electricity on the surface of the belt, and the printing medium is conveyed in this state.
In the inkjet printing apparatus installed in the electrostatic adsorption conveyance system, the adsorbability of the printing medium changes depending on the type of printing medium, usage environment conditions such as humidity, the printing medium conveying speed, soiling of the endless belt, and the like. A rapid change in adsorbability impairs the stability of printing medium conveying in the inkjet printing apparatus.
In order to stably convey a printing medium by adsorption to an endless belt, Japanese Patent Laid-Open No. 2004-262557 discloses that the cycle of the AC (+and −) applied to a power feed roller, which applies static electricity to the endless belt, is controlled depending on the type of printing medium. Also, Japanese Patent Laid-Open No. 2008-110853 discloses that the surface potential of an endless belt is detected, and the voltage applied to a power feeding means for applying static electricity to the endless belt is controlled in accordance with the detection result.
However, although the stable conveying of a printing medium can be realized in such inkjet printing apparatuses, the static electricity applied to the endless belt influences the discharge speed at which ink drops are discharged from the printing head. This results in disrupted landing of ink.
For example, in Japanese Patent Laid-Open No. 2004-262557, in the electric field generated by the charge on the endless belt and the charge on the printing medium, Coulomb's force acts on ink drops discharged from the printing head due to the charge of the ink drops. In other words, the behavior of ink drops discharged from the printing head is determined by the surface potential measured on the printing medium and the charge of the ink drops, and this fact leads to disrupted landing of ink drops.
Also, in Japanese Patent Laid-Open No. 2008-110853, the surface potential is uniformly “0” in the average static charge distribution on the belt directly below the printing head, but in the printing medium conveying direction, the surface potential is microscopically different in the positively charged portion, the negatively charged portion, and the boundary portions thereof. Accordingly, particularly in the case where the speed of the ink drops is low, the landing of ink drops is not constant due to variation in the discharge speed of the ink drops attracted due to the Coulomb's force. As a result, a line (having a pitch equal to half the positive/negative static charge cycle) appears in the portion where the polarity of the static electricity applied to the endless belt by the power feed roller switches between positive and negative.